1. Field of the Invention
The present invention relates to an image forming apparatus, and specifically relates to an image forming apparatus which forms an electrostatic latent image on the surface of a photosensitive member by an electrophotographic process and develops the latent image with toner.
2. Description of the Related Art
In an image forming apparatus of the electrophotographic type, two-component developers comprising a mixture of a carrier and a toner typically are used to develop an electrostatic latent image formed on the surface of a photosensitive member. When a two-component developer is used, the toner concentration Tc in the developer (the weight ratio of toner per total weight of developer) changes because toner alone is consumed in conjunction with image formation, such that a suitable amount of toner must be resupplied to the developer so as to maintain toner concentration Tc at a predetermined standard value.
Conventional toner replenishment control methods include well-known methods such as the ATDC method wherein the magnetic permeability of the developer is sensed via a magnetic sensor, or the amount of light reflected by the developer is detected by an optical sensor, so as to estimate the toner concentration Tc in the developer and resupply the required amount of toner. Other toner control techniques include AIDC methods, wherein the amount of light reflected by a toner test image formed on the surface of a photosensitive member under constant image forming conditions is detected by an optical sensor, to calculate the developing efficiency and estimate toner concentration Tc in the developer from the calculated developing efficiency, so as to resupply the required amount of toner.
While the ATDC method can be used in apparatuses which form full color images using toners of four colors, i.e., cyan, magenta, yellow, and black, disadvantages arise relating to black toner. That is, silica and the like are added to black toner to improve developing flow characteristics and improve image quality, but the bulk density of the toner fluctuates due to changes in humidity, thereby causing serious errors in toner concentration detection performed by magnetic sensors. Furthermore, black toner is commonly mixed with carbon black to enhance the deepness of its black color, but this mixing gives rise to other disadvantages when optical sensors are used because the spectral reflectivity characteristics approach that of the carrier.
Thus, the AIDC method has come to be used for toner replenishment of black toner.
In toner replenishment methods using AIDC, the amount of adhered toner of a toner test image is converted to a toner concentration using a table. Basically, this table is directly determined by data obtained experimentally using standard equipment. That is, a table is basically created using standard developing efficiencies which express the relationship between the toner concentration of the developer in a developing device and the amount of toner adhered to images developed by the developing device. Normally, copiers and printers have wide ranges of developing efficiencies due to errors introduced during manufacture. For example, the spacing between the photosensitive drum and the developing sleeve of a developing device can greatly affect developing efficiency, and small differences in this spacing appear from machine to machine. Accordingly, developing efficiencies will differ, i.e., the amount of toner adhered to the photosensitive drum will differ, from machine to machine even when the toner concentrations are identical. Thus, a disadvantage arises when the toner concentration is estimated by completely mechanical means using a table based on a standard developing efficiency, because of the effects of differences in developing efficiency from machine to machine.